The production of sulfuric acid from exhaust gases in industrial operations is normally conducted by a catalytic oxidation stage, where SO2 is converted to SO3 followed by a hydration step, whereby SO3 is converted to sulfuric acid vapor. The sulfuric acid is then condensed in a cooling step by indirect heat exchange with a cooling medium, normally air, in a so-called single wet condensation stage. The sulfuric acid is condensed directly, and therefore it is required that the temperature of the SO3 containing stream entering said cooling step is close to its dew point so as to avoid undesired acid mist formation. If the temperature is too high, the acid will not condense, and if the temperature is too low, an acid mist is formed which is difficult to trap. Another disadvantage of a process with a single wet condensation stage is that depending on the condenser design and process conditions some acid mist is still emitted from the condenser.
U.S. Pat. No. 5,455,015 describes a process for the conversion of SO2 to SO3, where subsequently SO3 in the wet gas or H2SO4 (H2O+SO3) is absorbed in a melt. The absorption of H2SO4 is conducted by bobbling the gas containing SO3 through the melt having a mixture of ammonium sulfate (NH4)2SO4 and ammonium bisulfate NH4HSO4 and which involves the reaction H2SO4+(NH4)2SO4→2 NH4HSO4. Ammonium bisulfate is then converted to ammonium sulfate by heating to above 350° C. or by electrolysis after which the sulfate is returned to the melt. Other sulfates disclosed are Na2SO4 and K2SO4. This patent is silent about acid mist formation and relies in the reaction of sulfuric acid with a sulfate to produce a bisulfate. The conversion of the bisulfate back to sulfate is not simple and requires expensive treatments such as thermal treatment at high temperature for instance. above 350° C. when using ammonium sulfate or electrolysis. In addition, the presence of sulfate in the acid-sulfate mixture is inexpedient, as the sulfate increases the melting temperature of the melt. Ammonium sulfate has a melting point of above 300° C. and already in a mixture of ammonium sulfate and ammonium bisulfate having only 20 wt % ammonium sulfate the melting point is above 200° C. (L. Cambi, G. Bozza, Ann. Chim. Appl. 13 (1923), 221).